The present invention relates to viscous dampers, and particularly, to rotary viscous dampers for retarding the downward closing of roll down doors such as fire doors.
It is known to provide a viscous speed governor to the shaft of a roll-up fire door to retard the unrolling speed of the fire door after actuation during a fire. Such a known fire door such as a KINNEAR.RTM. fire door is described in FIG. 1. These governors include a housing, a disk-like rotor within the housing and a cover enclosing the rotor within the housing. The housing can include notches arranged around an outside circumference forming lugs for engagement by a rachet-like device for preventing rotation of the housing in a first rotational direction during unrolling of the door, but allowing rotation in an opposite direction during subsequent roll-up.
U.S. Pat. No. 4,432,254, issued Feb. 21, 1984, describes a viscous damper using the shear film mechanism to retard rotation. Such a damper utilizes a rotor disk arranged rotationally within a housing having inside walls in close conformity with the opposite side surface of the rotor disk. A gap between the inside walls and the surfaces of the rotor disk is filled with a viscous fluid which resists relative rotation between the rotor disk and the housing by a shear film mechanism. Thus when the housing is fixed for zero rotation with respect to the fire door frame and the rotor disk is fixed for rotation with an axle shaft or reel of the fire door, unrolling of the fire door by rotation of the axle is resisted by the viscous damper.
Prior to the present invention, the damper and rotor must be sized per the weight of the fire door, requiring a plurality of sizes and resulting increased manufacturing and inventory costs.